This invention relates to hydraulic dampers for use in systems such as vehicle suspension systems or the like and, particularly, to hydraulic dampers of the kind including a dual tube type housing consisting of inner and outer tubes, a piston slidably disposed in the inner tube and dividing the interior thereof into two oil chambers, a piston rod secured to the piston and extending to the outside of the housing, a reservoir chamber defined between the inner and outer tubes and containing therein oil and gas, a passage connecting one of the oil chambers through which the piston rod extends with the gas containing portion of the reservoir chamber, and a valve member formed of a resilient material and having a lip portion acting as a one-way valve for allowing fluid flow in the passage only in the direction from the one oil chamber to the gas containing portion of the reservoir chamber.
In mounting the hydraulic damper on a vehicle or in operating the damper, gas will sometimes be contained or accumulated in the oil chambers according to the so-called aeration phenomenon or the like. When the oil chambers are not completely filled with oil the operating characteristics of the damper are substantially impaired. The one-way valve effectively returns any gas contained in the oil chambers to the reservoir chamber and maintains the desired characteristics of the damper and, usually, the valve opens in the extension stroke of the damper whereby the pressure in the one oil chamber through which the piston rod extends will increase with respect to the gas pressure in the reservoir chamber and closes in the contraction stroke of the damper whereby the pressure in the one oil chamber decreases with respect to the gas pressure.
The lip portion of the valve member of the one-way valve which engages with or separates from a cooperating valve seat during closing or opening the valve must have sufficient flexibility so as to open the valve easily and quickly and, also, must have sufficient stiffness so as to withstand the gas pressure in the reservoir chamber in the valve closed condition. However, it has been difficult to satisfy these contrary requirements and various proposals have been made with respect to the material, hardness, or thickness of the valve member and to the design and construction of the valve. For example, when the base portion of the lip portion is made of soft material or is formed to have a thin thickness so as to easily open the valve, the lip portion will deform excessively in the valve closed condition. When the stiffness of the base portion is increased the valve will not open easily.